1. Field of the Invention
The present invention relates to a heat-mode information recording medium and an information recording method which enable write (record) and read (playback) of information by use of high energy density laser light. More specifically, the invention relates to a heat-mode information recording medium, such as a recordable digital versatile disc (DVD-R), which is suitable for recording information by use of visible laser light.
2. Description of the Related Art
Information recording media on which information is recordable only once by use of laser light (optical discs) have been long known. The information recording media of such a type are referred to as recordable CDs (the so-called CD-R) as well. These discs have an advantage in that they can be rapidly delivered in low quantities at reasonable prices, compared with traditional CDs, and the demand for them has been growing as personal computers have come into widespread use recently. A typical structure of CD-R type of information recording media is a laminate in which a transparent disk-shape substrate, a recording layer containing organic dyes, a reflective layer made of metal such as gold and a protective layer made of resin are arranged in the order of mention.
And such optical discs can have information recorded thereon by being irradiated with laser light in the near infrared region (generally laser light of wavelengths in the vicinity of 780 nm) and undergoing local heat generation and deformation in their respective recording layers. On the other hand, reading (playback) of information from discs is generally performed by irradiating the discs with laser light of the same wavelength as the laser light used for recording has, and detecting differences in reflectivities between the areas deformed by heat generation (recorded area) and the areas remaining undeformed (unrecorded area) in the recording layers.
In recent years information recording media have been required to have higher recording densities. In order to heighten the recording densities, it is known to be effective that the diameter of laser light used for irradiation is narrowed down. Moreover, irradiation with laser light of shorter wavelengths is known to be theoretically more advantageous to further increase in recording density, because the diameter of laser light can be made more narrow the shorter the laser light is in wavelength. Therefore, the development of optical discs suitable for record and playback with laser light of wavelengths shorter than hither to used 780 nm has been pursued. For instance, optical discs referred to as recordable digital video discs (the so-called DVD-R) have been offered. These optical discs are each manufactured so as to have a structure that two disks which each have on a 120-mm-dia or 80-mm-dia transparent disk-shape substrate, wherein is formed a pregroove having a 0.8-μm track pitch smaller than 1.6 μm adopted as the track pitch of CD-R, a dye-containing recording layer, a reflective layer and a protective layer in the order of mention are bonded together with the recording layers inside, or one disk as described above and a disk-shape protective substrate having almost the same dimensions are bonded together with the recording layer inside. And the record and the playback of DVD-R are performed by irradiation with visible laser light (generally in the wavelength range of 600 to 700 nm). Therefore, DVD-R is considered to enable high-density recording, compared with CD-R type of optical discs.
As the information recording medium of DVD-R type can record information several times as much as that of a hitherto used information recording medium of CD-R type, it is desired that the information recording medium of DVD-R type has not only high recording sensitivity but also a low incidence of errors even in high-speed recording made out of necessity to process rapidly high-volume information in particular. In addition, it is desired to develop a recording layer capable of retaining its properties with stability for a long time even under exposure to light or heat, because a dye-containing recording layer is generally not so stable to prolonged exposure to heat or light.
JP-A-10-226170 discloses the DVD-R type of information recording medium having on a substrate a recording layer containing a certain cyanine dye. Therein, it is stated that the cyanine dye compound as specified can ensure high recording sensitivity and high reflectivity in the information recording medium. Further, JP-A-2001-287456 discloses the information recording medium having on a substrate a recording layer containing a cyanine dye of a specific structure, and states that such a cyanine dye enables the recording medium to have excellent recording characteristics and retain its recording characteristics with stability for a long time. On the other hand, JP-A-63-209995 discloses the CD-R type of information recording medium having an oxonol dye-containing recording layer provided on a substrate, and states that the use of such a dye compound enables long-term retention of steady record and playback characteristics. Therein, the oxonol dye compounds having ammonium ions introduced therein for forming salts are disclosed. In addition, JP-A-2000-52658 discloses the oxonol dye compounds providing optical information-recording media with high light resistance and durability as well as excellent recording characteristics.